The best way to lubricate bearings is to use a grease gun and pump grease into the bearing through a grease zerk or if you have to remove the bearings from the hub you can use a wheel bearing grease tool that you place the bearings in and then use a grease gun to pump grease through the bearing until the grease squeezes out of the bearings.
If you put too much grease in a bearing it can lead to buildup of heat, bearing seal damage and reduced bearing life.
You can also grease and lubricate bearings using your hand but it can be hard to get it done right if your not used to doing it by hand.
Some bearings also use oil instead of grease and if so there will often be an oil port where you pour oil into the hub or what is surrounding the bearing.
Too much grease in a bearing can lead to churning as well as friction and increase in pressure within the bearing cavity and lead to overheating and potential failure.
Grease is better for bearings than oil is although oil is sometimes used in some bearings instead of grease.
However some bearings such as those used on buses and large trucks and some machinery will use oil as a lubricant.
But in other bearings such as car and vehicle bearings grease is used and it doesn't leak out like oil does and provides better lubrication to the bearings.
Roller bearings should be greased if they are not sealed bearings.
If you have sealed roller bearings they are greased from the factory and require no further greasing.
However if the roller bearings are not sealed then they will require greasing to keep them in good condition.
The grease in the roller bearings helps reduce friction and reduces wear and also protects the surfaces of the roller bearings against corrosion and extends the bearings lifespan.
Greasing bearings that require grease including some roller bearings is crucial to ensure they don't prematurely wear out and seize up.
Without grease the roller bearings and other bearings would eventually seize up and could even break the shaft it's on.
The grease reduces the friction between the internal components and prevents metal to metal contact and ensures smooth movement.
Some roller bearings may require oil and others require grease as it depends on the roller bearing and what the roller bearing is on.
Some roller bearings are pre greased at the factory such as those used in some vehicle wheel bearing hubs or in industrial machinery.
In those cases the roller bearings don't require any additional greasing.
If there is a grease port then it requires grease and if there's an oil port then the roller bearing requires oil.
Increased friction, heat generation and accelerated wear can result from not enough or not any grease at all.